The invention concerns an accelerometer consisting of a transducing element and an encapsulation, which accelerometer has been designed for making accurate measurements of dynamic accelerations along a predetermined axis.
For the measurement of static and dynamic mechanical forces, electromechanical force-gauges are frequently utilized. In such gauges a change in electrical charge, voltage, current, or impedance is brought on by the forces to be measured.
Accelerometers, in which the acceleration of a seismic mass results in a mechanical force proportional to the acceleration are well known. The said force acts on a transducing element, e.g. a pressure-sensitive electrical resistor or semi-conductor or a piezo-electric element, resp. an electrodynamic system or any other known system for the conversion of mechanical energy into electrical energy.
The transducing elements may be acted on in the compression, bending, or shear modes and it has turned out that piezoelectric accelerometers working in the shear mode may be given advantageous properties, such as good linearity, low static and dynamic temperature coefficient, and an extended frequency range.
In a previously known construction of an accelerometer of the kind described, a base is provided, having a cylindrical stud, cemented or otherwise fastened to which there is a piezoelectric element shaped as a hollow cylinder, which element is surrounded by a seisic mass attached to it. The construction, being rotationally symmetric, ensures that the components of the accelerometer may be produced inexpensively, and with precision by turning and grinding. Furthermore the sensitivity to motion in directions perpendicular to the axis may be quite small. A weak point in this construction, however, lies in the poor bonding strength under temperature stresses of the joints between the cylindrical elements. The different temperature coefficients of expansion of these elements give rise to unwanted mechanical stresses.
In another, also known construction, the stud is shaped as a prism, the cross section of which is a regular polygon. The vertical faces of the prismatical stud carry flat transducing elements that are held in place by means of compression springs. Thus it is obtained that the stiffness of the stud is essentially the same in all directions perpendicular to the axis. A disadvantage of this construction lies in the difficulties in producing the stud and bonding it to the base plate when the size of the accelerometer is very small.